1. Field of the Invention
The present invention relates to a strapping machine, in particular, it relates to a strapping machine adapted to be capable of automatically or semi-automatically conducting a procedure which includes looping a band made of a tape of a thermoplastic resin such as a polypropylene around an object to be strapped, and fusion-bonding an overlapping portion of the band to effect strapping.
2. Description of the Prior Art
To realize heightened efficiency and energy savings in a strapping operation, a strapping machine has already been practically used in general which is adapted to be capable of conducting a procedure that includes looping a band made of a thermoplastic resin tape around an object to be strapped, tightening the band, fusion-bonding the resulting nodally overlapping end portion of the band under heat and pressure, and cutting the band.
In such a conventional strapping machine, a two-stage tightening is generally conducted as an operation for tightening a band, which comprises a primary tightening for high-speed/low-torque tightening of a band and a secondary tightening under low-speed/high-torque operation (see, for example, Japanese Examined Patent Publication No.13205/1992).
Of the conventional strapping machines, a strapping machine has been known which is provided with an electric motor such as an induction motor for driving cams such as a press, a heater, a slide and the like for strapping with a band to cause timely shifted rotations thereof, and an electromagnetic clutch interposed between the electric motor and each of the cams, thereby intermittently controlling the driving force of the electric motor by means of the electromagnetic clutch (see, for example, Japanese Examined Patent Publication No.69774/1991, and Japanese Unexamined Patent Publication No.58613/1989).
In such a conventional strapping machine, when the electric motor such as an induction motor is stopped by being locked via the electromagnetic clutch, an excessive temperature elevation is likely to occur due to a constant electrical current continuously applied to the electric motor, and a power circuit is required for driving the electromagnetic clutch, thereby leading to a complicated structure. Further, there is a problem that since each of the operation cams such as a press, a heater, a slide and the like has a cam profile with portions having an abruptly changing curvature, extra idle portions or the like, the cams undergo considerable impact due to the abruptly changed load and hence they are likely to undergo severe abrasion, leading to brief spans of lives of the cams, and, consequently, it is inevitably required to frequently carry out cumbersome maintenance operations such as the replacement of a cam. Moreover, there is a problem that since a large force is required for driving the cams, a large-sized cam driving motor is unavoidably used.